There are a variety of methods of molding articles. These include autoclaves, hydroclaves and compression molding. Particularly for high temperature molding (e.g., polyimide resins) one isostatic molding process has shown great advantage over other methods. This method is described in commonly assigned European Patent Application No. 87630010.4 entitled "Molding Method and Apparatus Using a Solid, Flowable, Polymer Medium" to Kromrey. An article is molded by contacting it with a solid polymer medium, such as an especially low strength unfilled silicone rubber which is solid and able to flow readily. One characteristic of such a silicone material is that it tends to coalesce under pressure so that the interfacial boundaries between the particles are so essentially conformed that the compressed rubber becomes translucent instead of opaque. (E.P.A. No. 87630010.4, Col. 10, lines 55-62) Thermal expansion of the medium or mechanical force is used to create molding pressure and thereby provides a substantially uniform pressure on the article precursor. Various temperature and pressure cycles can be attained; constant high pressures can be maintained on the article precursor during cooldown, optionally aided by flowing of medium to and from a vessel in which the article precursor is being molded. The method is particularly adapted to molding filler or fiber reinforced thermosetting polymer composite articles.
Typically, the above process cures composites or plastics over short periods of time. However, application of heat for rapid curing is often impractical or impossible. The higher temperatures typically needed for snap cures can cause severe warpage in laminates and can result in exotherms that can degrade polymers. An alternative to thermal curing is ultraviolet (UV) radiation curing. Although there can be a slight exotherm with a UV cure, the small exotherm can be controlled. Even in-depth UV curing does not result in the large thermal gradients that can develop with heat cures.
Recent advances in formulating UV curing polymers have resulted in high performance resin matrices for rapid curing composites. These polymeric materials must have adequate optical transparency required for in-depth curing. The optical transparency constraint also precludes the use of opaque tooling including some conventional vacuum bag materials, particularly for thick structures.
Accordingly, there is a constant search for UV composite curing processes.